I have this effect happen to me almost every time I heat water for
tea in the microwave oven. It isn't rare and it isn't hard to do (and
it was recently discussed in some depth on Physhare). What I have
found is that you have to get the water boiling vigorously before you
shut off the oven and remove the water. If you get it just boiling or
just before it boils, you don't get the effect, or if you get it it
isn't very dramatic. The fact that it must be boiling vigorously in
order to see the effect (at least in my experience) makes it hard for
me to believe the superheating hypothesis, although until I thought
about this fact, I was a proponent of that idea. Maybe someone can
explain to me how vigorously boiling water can be hotter than 100° C,
or if it can't be, how, once the boiling stops, the water can quickly
become hotter than it was when boiling and thus superheated.
Otherwise, it seems to me we need to look elsewhere for an
explanation.
I see this so often, that on those rare occasions when it does not
happen (Since I expect it, I always lower the tea bag into the water
carefully so it won't "erupt." I have had it erupt on me once, and
that it a gracious plenty. Fortunately, no serious damage but it sure
got my attention.), I am disappointed. But every time it doesn't
happen, it is because the water was just barely boiling when I took
it out.
Is Michael Edmiston's detailed explanation of this morning consistent
with my observation?